This is a very interesting question, and just to get my own head around it, I thought of the following example.
Take a €100,000 mortgage over 2 years at 40% interest
The repayments are a fixed €6118 per month. or €147k in total.
At the end of year one, you will owe €60,000.
Now take a €100,000 mortgage at 0%. The balance at the end of year 1 will be only €50,000.
So as the interest rate falls, there is a higher element of capital in each repayment. As the rates rise, there is a smaller element of capital, so the amount outstanding at any time will be higher.
I presume intersest rates have fallen since you took out the mortgage? If so, it seems odd that there is a balance due. It should be the other way round.
Brendan